An electronic ballast for a gas discharge, fluorescent or high-intensity discharge lamp provides the current, voltage, and wave-form conditions needed to operate the lamp. An instant-start ballast is a ballast that is designed to instantly start a gas discharge lamp such as a fluorescent lamp by applying a high voltage across the lamp without preheating the cathode. Prior art instant-start ballasts, like that shown in U.S. Pat. No. 5,925,990, have been in use for some time. In these prior art instant-start ballasts, it has been necessary to provide a relatively high output voltage to start the gas discharge lamps. In addition, the high output voltage needs to be maintained for an extended period of time, usually on the order of milliseconds, at a constant and well controlled value to ignite the lamps. In the case of the ballast of the '990 patent, the high start-up output voltage is sensed by placing a series resistor in series with a resonant capacitor. The start-up voltage is typically limited by a fast feedback loop and sensed for amplitude by rectifying it with a diode, smoothing it with a capacitor, and applying it to a sensing node of microprocessor.
Multiple lamp ballasts have been designed that provide power to two or more gas discharge lamps. These multiple lamp ballasts are configured to provide each one of the lamps coupled to the ballasts with the proper operating current and voltage. For example, the above described instant-start ballast configuration has been adapted for use with a four-lamp ballast as set forth in the prior art circuit shown in FIG. 1. In such a circuit, a single ballast 2 essentially drives two complete output stages 4 and 6 of the kind previously described above, with each output stage 4 and 6 providing output terminals 8 and 10 for two respective lamps. A transistor 12 is used to invert the signal from the microprocessor 3 for one of the output stages 4 and 6. Inverting the signal for one of the output stages 4 and 6 reduces the ripple current stress on the bulk capacitors 14 and 16 by causing the two output stages 4 and 6 to run out of phase. This prior art system works but has the disadvantage of being relatively expensive. The added expense is a result of the need to use two half-bridge drivers 18 and 20 and four output FETS 22, 24, 26 and 28, along with two voltage limiting circuits 30 and 32. The increased number of components used in such a prior art circuit also decreases the reliability of the circuit.
Relatively high output T5 lamps are gradually replacing older T8 lamps in mainstream office ceiling applications. The most efficient way of driving these lamps is using instant-start technology. However, because of the high lamp voltage and high lamp current of these lamps, the conventional parallel resonant, instant-start circuits for such are very inefficient and expensive to make. For the greatest efficiency, series-resonant, direct coupled ballasts are preferred. However, these types of ballasts have problems in meeting the UL through lamp leakage requirements.
In view of the above described deficiencies in the prior art, what is needed is a more cost effective and efficient method and circuit for implementing an instant-start, four-lamp ballast that uses fewer components than prior art ballasts.